1. Field of the Invention
The present invention relates to microwave amplification tubes, such as a traveling wave tube (TWT) or klystron, and, more particularly, to a coupled cavity microwave electron tube that produces a broadband response at high frequencies.
2. Description of Related Art
Microwave amplification tubes, such as TWT's or klystrons, are well known in the art for enabling a radio frequency (RF) signal and an electron beam to interact in such a way as to amplify the power of the RF signal. A coupled cavity TWT typically includes a series of tuned cavities that are linked or coupled by irises (also know as notches or slots) formed between the cavities. A microwave RF signal induced into the tube propagates through the tube, passing through each of the respective coupled cavities. At relatively high frequencies (e.g., around 100 GHz), a typical coupled cavity TWT may have a hundred or more individual cavities coupled in this manner. Thus, the TWT appears as a folded waveguide in which the meandering path that the RF signal takes as it passes through the coupled cavities of the tube reduces the effective speed of the signal enabling the electron beam to operate effectively upon the signal. Thus, the reduced velocity waveform produced by a coupled cavity tube of this type is known as a “slow wave.”
Each of the cavities is linked further by an electron beam tunnel that extends the length of the tube and through which an electron gun projects an electron beam. The electron beam is guided by magnetic fields that are induced into the beam tunnel region. The folded waveguide guides the RF signal periodically back and forth across the drifting electron beam. Thus, the electron beam interacts with the RF signal as it travels through the tube to produce the desired amplification by transferring energy from the electron beam to the RF wave.
The magnetic fields that are induced into the tunnel region are obtained from flux lines that flow through polepieces from magnets lying outside the tube region. The polepiece is typically made of permanent magnetic material, which channels the magnetic flux to the beam tunnel. This type of electron beam focusing is known as Periodic Permanent Magnet (PPM) focusing. The iron polepieces extend directly into the interaction region between the RF signal and the electron beam, thereby forming an integral part of the folded waveguide circuit. The introduction of the polepieces into the circuit serves two purposes. First, it increases the stability parameter (λp/L) of the magnetic focusing field for the beam, thereby reducing the beam voltage requirements for operation at the same frequency and output power. Second, it facilitates the efficient transfer of heat out of the circuit by allowing the circuit to be made of solid copper in the orthogonal transverse region, making the overall design more robust and suited for harsh operating environments, such as in certain military applications.
Klystrons are similar to coupled cavity TWTs in that they can comprise a number of cavities through which an electron beam is projected. The klystron amplifies the modulation on the electron beam to produce a highly bunched beam containing an RF current. A klystron differs from a coupled cavity TWT in that the klystron cavities are not generally coupled. A portion of the klystron cavities may be coupled, however, so that more than one cavity can interact with the electron beam. This particular type of klystron is known as an extended interaction klystron.
For a coupled cavity circuit, the bandwidth over which the amplification of the resulting RF output signal occurs is typically controlled by altering the dimensions of the cavities and coupling irises. The power of the RF output signal is typically controlled by altering the voltage and current characteristics of the electron beam. There is an inverse relationship between the frequency of the RF output signal and the size of the cavities. In other words, where it is desired that the coupled cavity circuit propagate higher frequencies, the cavity size for the circuit must be made smaller. On the other hand, for the coupled cavity circuit to propagate more frequencies, the coupling iris size must be made larger.
Typically in order to maximize the magnetic flux transported to the beam tunnel by the polepieces, the polepieces are made as thick as possible and the cavities are located between the polepieces. But, as operating frequencies become higher, one must reduce the thickness of the polepieces if cavities are to be placed between them. This results in reduced flux in the beam tunnel, reduced beam power and lower RF output power. A coupled cavity circuit that propagates higher and more frequencies at higher power would be advantageous. Accordingly, for high power applications, it would be desirable to provide a coupled cavity circuit that utilizes thicker polepieces in order to utilize higher power electron beams, while at the same time maintaining the desired size and number of cavities between the polepieces.